1. Field of the Invention
This disclosure relates to a temperature-compensated crystal oscillator, and more particularly, to a temperature-compensated crystal oscillator capable of improving temperature compensation accuracy even in a high temperature without increasing a sensitivity of a voltage controlled oscillator.
2. Description of Related Art
<Temperature-Compensated Crystal Oscillator in Prior Art: FIG. 4>
A temperature-compensated crystal oscillator (TCXO) in prior art will be described with reference to FIG. 4. FIG. 4 is a circuit block diagram illustrating a temperature-compensated crystal oscillator in prior art. The TCXO in prior art includes an oscillator circuit 1 and a crystal resonator 2 as illustrated in FIG. 4.
The oscillator circuit 1 includes an input terminal (AFC terminal) that receives an external reference signal, an automatic frequency control unit (AFC: Automatic Frequency Control) 11, an oscillator unit (OSC) 12, an output buffer unit (OUT BUFFER) 13, a temperature sensor unit (TEMP SENSOR) 14, a temperature compensation unit (FUNC) 15, a nonvolatile memory (NVM) 16, and a constant voltage power source (REG) 17.
The automatic frequency control unit 11 controls a sensitivity (voltage gain) for the external reference signal input from the AFC terminal and outputs it to the oscillator unit 12. The oscillator unit 12 outputs a signal oscillating using the crystal resonator 2 and the sensitivity-controlled voltage (AFC) to the output buffer unit 13. The output buffer unit 13 buffers (amplifies) the oscillation signal from the oscillator unit 12 and outputs it to the output terminal (OUT terminal). Here, the output buffer unit 13, the crystal resonator 2, and the oscillator unit 12 constitute a voltage-controlled crystal oscillator (VCXO).
The temperature sensor unit 14 measures an ambient temperature of the crystal resonator 2 and outputs it to the temperature compensation unit 15. The temperature compensation unit 15 is a circuit for generating a function. The temperature compensation unit 15 reads a parameter for temperature compensation stored in the nonvolatile memory 16, performs computation based on the parameter and the measurement temperature value input from the temperature sensor unit 14, and outputs a temperature-compensated voltage to the oscillator unit 12. The oscillator unit 12 supplies the signal oscillating using the crystal resonator 2 along with the signal from the AFC 11 and the compensation voltage output from the temperature compensation unit 15.
<Circuit of Temperature-Compensated Crystal Oscillator in Prior Art: FIG. 5>
Next, a circuit of the temperature-compensated crystal oscillator in prior art of FIG. 4 will be described with reference to FIG. 5. FIG. 5 is a circuit diagram illustrating the temperature-compensated crystal oscillator in prior art. Comparing FIGS. 4 and 5 regarding the temperature-compensated crystal oscillator in prior art, the crystal resonator 2 corresponds to the crystal resonator X. The output buffer unit 13 corresponds to the buffers 32 and 33, the resistor R3, and the capacitor C3. The oscillator unit 12 corresponds to the inverter IC31, the resistors Rf, R1, and R2, the capacitors C1 and C2, and the variable capacitance diodes VD1 and VD2.
In FIG. 5, the automatic frequency control unit 11 and the constant voltage power source 17 of FIG. 4 are omitted. It is noted that the output from the automatic frequency control unit 11 is input to the terminal V1T of FIG. 5.
In the aforementioned temperature-compensated crystal oscillator in prior art, typically, temperature compensation is performed within a range of −40 to +85° C. In recent years, in Europe, an act for compelling an emergency call such as e-call was legislated, and introduction of the related equipment is in progress. For use in a vehicle, the TCXO is required to operate across a wide temperature range, for example, −40 to +105° C.
There is no problem in a typical temperature compensation range (i.e., −40 to +85° C.). However, there are some problems as described below in a wide temperature range (e.g., −40 to +105° C.), particularly, a temperature range of +86 to +105° C.
<Example of Temperature Compensation in Prior Art: FIGS. 6A-6D>
An example of temperature compensation in prior art will be described with reference to FIGS. 6A to 6D. FIGS. 6A to 6D are explanatory diagrams illustrating an example of temperature compensation in prior art. In FIGS. 6A to 6D, the abscissa denotes a temperature (Temp), and the ordinate denotes a frequency characteristic in FIGS. 6A and 6D or a voltage characteristic in FIGS. 6B and 6C. FIG. 6A illustrates a characteristic of the “oscillation frequency Fout without temperature compensation.” This characteristic is a frequency characteristic depending on the temperature characteristics of the crystal resonator and the oscillator circuit.
FIG. 6B illustrates a voltage characteristic of an ideal temperature compensation voltage V1, and FIG. 6C illustrates a voltage characteristic of a realistic temperature compensation voltage V1. If the voltage characteristic V1 for the ideal temperature compensation voltage of FIG. 6B is applied to the oscillation frequency of the voltage-controlled crystal oscillator without temperature compensation of FIG. 6A, the oscillation frequency after the ideal temperature compensation becomes flat.
However, a deviation occurs in a high temperature area between the frequency characteristic for the ideal temperature compensation voltage and the frequency characteristic for the realistic temperature compensation voltage. Due to such a deviation, if the voltage characteristic V1 for the realistic temperature compensation voltage of FIG. 6C is applied to the oscillation frequency of the voltage-controlled crystal oscillator without temperature compensation of FIG. 6A, the oscillation frequency after the realistic temperature compensation of FIG. 6D basically becomes flat, but it abruptly rises in a high temperature area. That is, in the high temperature area, the temperature compensation is not normally performed.
In addition, in the TCXO in prior art, a power voltage of the TCXO has been lowered by highly integrating the employed integrated circuit (IC) and advancing miniaturization in the process. For example, the power voltage has been lowered from 5 V to 3.3 V and from 3.3 V to 1.8 V.
<Related Techniques>
As the related techniques, there are disclosed: “Temperature-compensated Piezo-oscillator” in Japanese Patent Application Laid-open No. 2004-104609, assigned to TOYO Communication Equipment Co., Ltd.; and “Temperature-compensated Piezo-oscillator” in Japanese Patent Application Laid-open No. 2005-033329, assigned to CITIZEN WATCH Co. Ltd.
Japanese Patent Application Laid-open No. 2004-104609 discloses a temperature-compensated piezo-oscillator in which a temperature variation is applied to the variable capacitance diode as a voltage variation by integrating a variable capacitance diode into a frequency/temperature compensation circuit, a capacitance is changed based on that voltage such that the capacitance decreases to increase the frequency, and the capacitance increases to decrease the frequency.
Japanese Patent Application Laid-open No. 2005-033329 discloses a temperature-compensated piezo-oscillator including a crystal oscillator circuit having a low-temperature MOS capacitance element and a high-temperature MOS capacitance element connected to each other in parallel, a low-temperature bias signal generating circuit, and a high-temperature bias signal generating circuit, so that temperature compensation for a low-temperature area and temperature compensation for a high temperature area are independently performed.
However, in the TCXO in prior art, as the power voltage is lowered, an internal voltage is reduced, and a dynamic range of the circuit voltage is narrowed. As a result, a temperature compensation voltage range for the voltage applied to the voltage-controlled crystal oscillator (VCXO) is narrowed, and a temperature range that can be compensated is narrowed.
In this regard, it is conceivable that this problem may be addressed by increasing a frequency-to-voltage sensitivity of the VCXO. However, in such a countermeasure, a noise sensitivity also increases, so that a phase noise as an important characteristic required in the TCXO may be degraded.
Both the temperature-compensated piezo-oscillators disclosed in Japanese Patent Application Laid-open Nos. 2004-104609 and 2005-033329 are designed to perform temperature compensation using a high-temperature compensation circuit in a high temperature and using a low-temperature compensation circuit in a low temperature. However, it is difficult to operate both the circuits in combination and obtain an easy and simple configuration.